fix: stop reader key navigation leaking behind open overlays

Arrows/j/k/Home/End bubbled to the reader's window handler while a
jump/settings/action/collections/tags/context overlay was open, paging the
chapter behind it (FE-1); native select/contenteditable weren't exempt (FE-4).
New unit-tested isTypingTarget() + an overlay-open early return fix both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -3,6 +3,7 @@
     import { browser } from '$app/environment';
     import { afterNavigate, goto, invalidateAll } from '$app/navigation';
     import { fileUrl, ApiError } from '$lib/api/client';
+    import { isTypingTarget } from '$lib/readerKeys';
     import { GAP_PX, type ReaderPageGap } from '$lib/api/preferences';
     import { preferences } from '$lib/preferences.svelte';
     import { updateReadProgress } from '$lib/api/read_progress';
@@ -675,9 +676,9 @@
     );
 
     function onKeydown(e: KeyboardEvent) {
-        // Don't hijack keys while the user is typing in an input.
-        const target = e.target as HTMLElement | null;
-        if (target && (target.tagName === 'INPUT' || target.tagName === 'TEXTAREA')) return;
+        // Don't hijack keys while the user is typing or focused on a native
+        // control (input/textarea/select/contenteditable own their own keys).
+        if (isTypingTarget(e.target)) return;
         // `?` opens the keyboard-shortcut help overlay. While it's open,
         // swallow every key here so paging can't happen behind it — the
         // Sheet owns its own Escape/close.
@@ -687,6 +688,20 @@
             return;
         }
         if (shortcutsOpen) return;
+        // Any other overlay (jump/settings/action/collections/tags/context menu)
+        // owns its own keys; swallow reader paging so arrows/j/k/Home/End can't
+        // act on the chapter *behind* the overlay. Those overlays only
+        // stopPropagation() on Escape, so navigation keys otherwise bubble here.
+        if (
+            chapterJumpOpen ||
+            settingsOpen ||
+            contextMenuOpen ||
+            actionSheetOpen ||
+            collectionsModalOpen ||
+            tagsModalOpen
+        ) {
+            return;
+        }
         // Esc always exits fullscreen if active — applies in both
         // modes, including when the bars are hidden. Handled before
         // the per-mode switches so it doesn't get shadowed.